Abstract

There are many endogenous signaling and regulatory molecules which can infuence the growth, development and physiology of plants. Hormones are produced specif cally for signaling. They are often transported from sites of synthesis to distant sites of action and they operate at very low concentrations. In contrast, some other chemicals may provide signals, but it may not be their main function or activity, such as primary metabolites, reactive oxygen species (ROS) and inorganic ions, and they often act locally within individual cells. The hormone family includes auxins, cytokinins (CK), gibberellins (GA), abscisic acid (ABA), ethylene (ETH), brassinosteroids (BR), strigolactones (SL), salicylic acid (SA), jasmonates (JA), and peptides. They are synthesized from common metabolic precursors, but use specialized pathways, and their production is very strictly controlled, both spatially and temporally. All hormones infuence multiple aspects of plant function, and they influence the synthesis and actions of each other. The interactions between hormones, environmental signals, and developmental programs are so complex that the description and modeling of the whole system is very challenging. Some hormone receptors are membrane anchored (CK, ETH, BR, and peptides) while others are soluble (auxin, GA, ABA, SA, jasmonic acid (JA), and SL). Co-receptor complexes are formed during perception of hormones including auxin (with IAA/ AUX transcriptional repressor proteins), JA (with JAZ transcriptional repressor proteins), and ABA (with phosphoprotein phosphatase PPC2). Hormone perception can lead to signal transduction through protein phosphorylation cascades (e.g., ABA, CK, BR, and peptides). Other hormone-receptor complexes trigger interaction with F-box proteins and ubiquitination enzymes that target proteins such as transcriptional repressors for degradation by the 26S proteasome (e.g., auxin, GA, SA, JA, and SL). Such signaling changes protein activities and gene transcription, with consequent changes to plant development and physiology. The effects of hormones are so profound that through breeding and agrochemical approaches in the 20th century, they gave us high-yielding, nutritious and resilient crops. In the 21st century we look to plant hormones to help meet the increasing demand for food production under ever-more challenging environmental conditions.